An advanced display technology employs the use of micro light-emitting diodes (μLEDs). μLED display technology is expected to outperform OLED and potentially be one of the contenders to replace LCD display technology as the predominant display technology in the next decade. However, the development for mass production of μLED displays has reached a bottleneck due to complications with the mass transfer of μLEDs from a source substrate, such as a growth wafer, to the target display substrate. Transfer yields and efficiency are currently too low for mass production of large displays to be feasible. Large displays, such as tablets or televisions, require millions of μLEDs to be transferred from the source wafer onto the display substrate, and with transfer yields less than 100%, dead pixels are becoming a significant issue in manufacturing a display with the requisite number of μLEDs.
One of the leading pick and place devices used for transferring μLEDs is an elastomeric stamp, such as the stamp and transfer method described in WO 2016/012409 (Bower et al., published Jan. 28, 2016). The device includes a polymer stamp that has a rate-dependent adhesion property allowing μLEDs to be picked up and placed down by varying the speed at which the stamp is moved. WO 2016/116889 (Chaji et al., published Jul. 28, 2016) describes a method of a direct transfer between a donor substrate and a receive substrate via employing a combination of attractive, repulsive and weakening forces. WO 2013/119671 (Bibl et al., published Aug. 15, 2013) discloses a stamp-like transfer head that operates to pick up and place micro devices using electrostatic forces.
FIG. 17 is a drawing exemplifying these conventional stamp-based methods, such as using an elastomeric stamp or electrostatic stamp transfer head. FIG. 17 depicts a stamp 200 of a having an array of a plurality of transfer heads 202. The stamp 200 picks up an array of micro devices 204 (e.g., μLEDs) from a source wafer 206, by operation of electrostatic, elastomeric, or other like forces applied via the transfer heads 202. The stamp 200 can then move the micro devices 204 and bond them to a display substrate 208. The micro devices 204 are released to deposit on the display substrate 208 by manipulating the associated forces.
As seen in FIG. 17, a resolution of the source wafer 206, i.e., the number of micro devices 204 per unit area, typically is significantly greater than a resolution of the display substrate 208. Both the electrostatic and the elastomeric stamps pick up a portion of the micro devices 204 from source wafer 206 at the resolution of the display substrate 208 to permit placement at the resolution of the display substrate 208. Accordingly, because the stamp can only pick up and release a common fixed number of elements at the same time, the stamp 200 must have the resolution of the target display substrate 208, and thus must make many trips between the growth wafer 206 and the display substrate 208. FIG. 17 illustrates three exemplary stamp passes, and the wafer must be repeatedly stamped in this fashion until the display device includes the requisite number of micro devices. This provides for an inefficient method of picking up and placing micro devices for μLED displays that require millions of μLEDs.
In a wholly separate field of technology, micro-electromechanical systems are microscopic devices with moving parts. The use of micro-electromechanical systems (MEMS) involving a flexible membrane, actuated by applying a potential difference between the membrane and an electrode, is well known. These flexible membrane MEMS have mainly been demonstrated in acoustic devices such as a MEMS microphone, or a MEMS ultrasound scanner (e.g., McMullen et al., GB 2469412, published Oct. 13, 2010), both of which involve the acoustic vibration of the flexible membrane.
Efforts in MEMS design have been focused on the design of suitable structures for acoustic generation as well as on the implementation of effective driving methods. For example, Digital Sound Reconstruction (DSR) theory, as described for example in Gabriel et al., US2003/0044029 (published on Mar. 6, 2003) offers low distortion and high linearity to sound production. To be effective, DSR requires a high number of identical speakers that can be individually controlled. Research efforts thus also have focused on the production of miniaturized devices with homogenous and well-tuned properties. In Loeb et al., U.S. Pat. No. 6,829,131B1 (issued Dec. 7, 2004), an acoustic transducer with a diaphragm formed on a single silicon chip using CMOS-MEMS technology is disclosed. With such configuration it is then possible to obtain higher integration and uniformity within an array of devices.
Cohen et al., U.S. Pat. No. 8,780,673B2 (issued Jul. 15, 2014) and Cohen et al., WO2014141258A1 (published Sep. 18, 2014) disclose an actuation system comprised of an array of identical elements constrained to move along one direction and actuated by electromagnetic and electrostatic forces respectively. The single elements are not individually controlled, and a driving method controlled by an active matrix is not disclosed.
In the literature, MEMS structures composed of electrostatically actuated membranes comprising multiple electrodes are reported in the field of RF switches and varactors. Examples of such devices are disclosed in Chou, US2006/0226501A1 (published Oct. 12, 2006), Lan et al., U.S. Pat. No. 8,363,380 (issued Jan. 29, 2013) and Breen et al., U.S. Pat. No. 8,849,087B2 (issued Sep. 30, 2014).
The above fields, however, have offered only limited uses of the capabilities of MEMS devices.